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Homework Help: Finding change in internal energy

  1. Nov 12, 2013 #1
    1. The problem statement, all variables and given/known data

    In exercising, a weight lifter loses 0.150 kg of water through evaporation, the heat
    required to evaporate the water coming from the weight lifter's body. The work done
    in lifting weights is 1.40x105 J. (a) Assuming that the latent heat of vapourisation
    of perspiration is 2.42x106 Jkg-1, find the change in the internal energy of the weight lifter.

    2. Relevant equations

    Q = mL

    Δu = Q - w

    3. The attempt at a solution

    Q = mL
    = 0.15 * 2.42x106
    = 363000 J

    This is the amount of energy required to vapourise the perspiration.

    Δu = Q - w

    = 363000 - 1.40x105

    = 223000 J

    Is this correct?

    I was half expecting my answer to be negative since the weight lifter will have less energy after lifting weights, but I suppose it makes sense that he would also get hot which is what internal energy measures.

    ETA The next part of the question goes on to say this;

    Which supports my feeling that i'm wrong...

    One thought i'm having is that since the energy required to vapourise the perspiration is coming from the body, I should be giving it as a negative figure.
    Last edited: Nov 12, 2013
  2. jcsd
  3. Nov 12, 2013 #2
    Q in the first law expression is defined as the heat gained by the body. In your example, Q should be negative because evaporation of sweat causes the body to lose heat.
  4. Nov 12, 2013 #3
    I thought that would be the case, thanks.
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